Before a data link is set up, mobile radio receivers are synchronized to the transmission and reception clock in one or more base stations. This is generally achieved by means of a three-step method: the slot synchronization (time slot synchronization) is carried out in a first synchronization step. When the slot timings are known, the frame boundaries of the signal are determined in a second synchronization step (frame synchronization). The scrambling code that is being used by the transmitter (base station) is identified in a third synchronization step.
The frequency synchronization and channel estimation which is carried out in order to measure the transmission response of the adaptive mobile radio channel are generally carried out only after the three initial synchronization steps that have been mentioned are complete.
Third-generation mobile radio systems, for example 3GPP (3rd Generation Partnership Project), envisage the use of transmitter-end antenna diversity (so-called TX antenna diversity), which is also referred to in the following text as TX diversity. Transmission-end antenna diversity means that the transmission signal is transmitted from at least two different antennas. If two antennas are used, the emitted signal on at least one of the two antennas is modulated with a specific signal sequence, so that the two transmission signal streams are transmitted simultaneously and orthogonally with respect to one another. Transmitter-end antenna diversity allows the performance of the data transmission system to be significantly improved by means of antenna-specific demodulation when the data is received in the mobile station.
For this purpose, however, the receiver (mobile station) has to know whether a TX diversity method is being used and, if appropriate, which method is being used. In consequence, it is necessary to detect the TX diversity mode as early as possible and as reliably as possible in order to ensure efficient data reception.
Three fundamentally different approaches to solve this detection problem are known from the prior art:    (1) A first option is for the base station to use a monitoring channel to signal to the mobile station that TX diversity is or is not being used. The BCH (Broadcast Channel) can be used as the monitoring channel in UMTS (Universal Mobile Telecommunications System). A method such as this, which is also referred to as layer 3 (L3) signalling, is described in “An alternative scheme to detect the STTD encoding of PCCPCH”, Texas Instruments, TSG-RAN WG1 meeting #3, 150, Nynashamn, Sweden 22-26, 16 Mar. 1999.    (2) A second option is to verify transmission antenna diversity by detection of an indicator sequence that is modulated symbol-by-symbol on the synchronization channel. In contrast to the method according to (1), there is no need to use a monitoring channel for notification of the receiver. This method is described in “Fast reliable detection of STTD encoding of PCCPCH with no L3 messaging overhead”, Texas Instruments, TSG-RAN WG1 meeting #4, 372, Yokohama Japan, 18-20 Apr. 1999.    (3) A third option is to carry out blind detection of the second TX transmission antenna using pilot sequences (in UMTS, for example, the pilot sequence that is transmitted via the CPICH channel (Common Pilot Channel)). A method such as this is described in “STTD encoding for PCCPCH”, Texas Instruments, TSG-RAN Working Group 1, meeting #2, 83, Yokohama 22-25 Feb. 1999. This report proposes that the received data symbols be split on a transmission antenna specific basis (that is to say based on the hypothesis that TX diversity is being used in the transmitter) and that the received pilot symbols then be added coherently (that is to say taking into account the magnitude and the phase) channel-by-channel (that is to say for each transmission antenna). The ratio for the addition result for the main transmission antenna to the addition result for the diversity antenna (whose presence/absence is intended to be tested) is then formed. The determined ratio is subjected to a threshold value comparison. If the ratio is higher than the threshold value, it is assumed that antenna diversity is not being used at the transmitter end. Otherwise, the hypothesis that transmitter-end antenna diversity is being used is confirmed.
The methods described in (1) and (2) require frequency synchronization and knowledge about the transmission channel (carrying out a channel estimation process) in order to obtain acceptable detection results. This additional processing effort and time penalty adversely affects the performance of the initial synchronization. The methods (1) and (2) can therefore generally not be used.
The difficulty with the method described in (3) is that its performance deteriorates very rapidly as the frequency error between the base station and the mobile station increases. A frequency error of more than 1 kHz (0.5 ppm) at a carrier frequency of 2 MHz is sufficient to cause considerable performance degradation. A frequency error of about 4 kHz (1.9 ppm) would result in the method described in (3) detecting a second “virtual” TX transmission antenna with the same probability as the first “real” transmission antenna, even though only the first transmission antenna is being used for transmission (that is to say there is no transmitter-end antenna diversity). Since frequency errors of about 3 ppm may occur during the initial synchronization in practice, the validity of the method according to (3) is sufficiently good only if frequency synchronization has previously taken place between the transmitter (base station) and receiver (mobile station). Early TX diversity detection is thus not possible, even by using this method.